By using the given variables, we can customize the BoldBI dashboards to meet your requirements. Follow the steps below to create a custom theme file:
Download the default light and dark files from the Github repository and extract them.
Copy all CSS files from the light folder inside the themes folder and paste them into the newly created folder. For example, create a new folder named darkviolet.

boldbi.application.definition.css and boldbi.core.definition.css files in the editor, and then you will be able to customize the color code in the provided variables. For example, customize the variables as follows.

README.md on the Github repository to generate boldbi.theme.definition.min.css.
README.md file, you can find the theme files in the directory {src directory}/assets/themes/{theme directory name}/boldbi.theme.definition.min.css.